Brake Booster/Master Cylinder Options

[varjack]

the brakes feel funny in my newly-rebuilt 240 turbo rally car. the master cylinder and booster are new, and the brakes have been vacuum bled to the point that they work, but they're wildly inconsistent. at a rallycross I'd push the pedal and it felt like the brake pressure increased as the car slowed down. it was really hard to lock down good braking as sometimes I'd push the pedal and the brakes would barely work, and then other times the wheels would lock up with similar amounts of pedal pressure.

per the vacuum/boost gauge, at idle i'm pulling about 25 in/hg vacuum and boost goes up to 12psi. someone mentioned that as i'm slowing down and vacuum increased, it made the boost assist stronger without having to push the pedal harder, but now that I think about that I'm not sure because when I lift off and the BOV opens the gauge shows vacuum swinging in the other direction pretty quickly. additionally, if I'm idling and stab the brakes idle shoots up about 600 rpm for a sec before settling.

im not sure where to go from here. do I buy another booster? a hydroboost is an option, but that just adds complexity as I'd probably need a better power steering pump and even then I've heard under load you lose power steering assist. some people swear by manual brakes in race cars, but I see I'd have to do something custom and I don't have a welder. although I do like the sound of manual brakes. less weight, better feel, takes up less space, one less part to fail.

theres the sellhom bromsbox, which would need a custom mount. chase bays makes some nice stuff but not for the 240, so that'd have to be custom too. or wilwood. disregarding the custom mount and pedal mods, if I ran a dual-cylinder setup, how much would I have to change the triangular brake line routing? do I need to factor in new brake lines, or is there a solution as simple as getting a mounting plate/cutting some holes in the firewall and the pedal to increase the leverage?

 

[VB242]

Do you have a check valve in the brake booster vacuum line? It should allow only vacuum one way to the booster and keep the boost psi's out.

 

[varjack]

Do you have a check valve in the brake booster vacuum line? It should allow only vacuum one way to the booster and keep the boost psi's out.

yeah I do. even checked it against the other one I had and it seemed to hold pressure better.

 

[dbarton]

yeah I do. even checked it against the other one I had and it seemed to hold pressure better.

I had inconsistent vacuum assist for a long time, but I attributed mine to the low compression B21FT (7.5:1) plus a bigger cam. I added a vacuum pump with vacuum sensor similar to what the mid-2000s Volvos used. It keeps vacuum at a great level and only powers on then vacuum drops, like if I touch the brake pedal. Works really well. I wrote all about it here: https://www.240turbo.com/volvo240bigbrakes.html#vacuumpump

 

[culberro]

the brakes feel funny in my newly-rebuilt 240 turbo rally car. the master cylinder and booster are new, and the brakes have been vacuum bled to the point that they work, but they're wildly inconsistent.

My guess is the front wheel bearings are loose or you are bending the spindles. Either way I think you’re getting pad knock back. It can also happen with the rear axles if they are loose and have some plunge to them. Left foot brake and give the brakes a few light presses after a long straight section to bring the pads back into contact with the rotor and firm up the pedal.